The present invention relates to the catalytic reduction of nitrogen oxide emissions from fossil fueled power plants and more particularly to the control of the flue gas temperature entering the catalytic reactor during low load operation.
Three classes of emissions from fuel-burning processes are judged significant from an air quality standpoint. These are particulate matter, sulfur oxides and nitrogen oxides. Historically, the particulate matter received the greatest attention. This was then followed by the sulfur oxides because of the possible health effects and from its potential to damage vegetation and property. However, in recent years, the oxides of nitrogen have become of increasing concern because they participate in complex chemical reactions that lead to the formation of photo-chemical smog. Also, both the sulfur oxides and nitrogen oxides have been implicated as precursors to acid rain.
The reduction of nitrogen oxide emissions has taken two tacks, in-furnace control and post-combustion control. The in-furnace control involves such techniques as gas recirculation, low excess air firing, concentric tangential firing and overfire air. The post-combustion control primarily involves a reductant and catalyst to reduce nitrogen oxides to nitrogen gas and water vapor.
One particular system for the catalytic reduction of nitrogen oxides (NO.sub.x) is referred to as selective catalytic reduction. This uses a catalyst and a reductant, ammonia gas to dissociate NO.sub.x to nitrogen gas and water according to the following reactions: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O EQU `bNO.sub.2 +4NH.sub.3 +O.sub.2 .fwdarw.3N.sub.2 +6H.sub.2 O
Since NO.sub.x is approximately 95 percent NO, the first reaction dominates.
The ideal operating temperature range for selective catalytic reduction is generally from 300.degree. to 400.degree. C. (572.degree. to 752.degree. F.). When operating conditions fall much below 300.degree. C., the potential for ammonium bisulfate formation and sulfur trioxide deposits on the catalyst surface increases. This can cause permanent catalyst activity loss. Above 400.degree. C., ammonia gas may dissociate reducing the effectiveness of the process. If temperatures were to exceed about 450.degree. C. (842.degree. F.), the catalyst activity might be permanently impaired due to sintering.
The catalytic reaction chamber is typically located in the flue gas stream between the outlet from the economizer section and the flue gas inlet to the air preheater. This normally provides a flue gas temperature to the catalytic reactor within the above-noted operating conditions. Insufficient gas temperature occurs during low load operation.